This grant concentrates on endoplasmic reticulum (ER) protein misfolding and ER stress-induced endocrine cell death, using the thyroid gland as a model. Diseases of this kind affect every organ system. The thyroid is an ideally-suited model system in which to study this problem because, unlike the situation in pancreatic beta cells (in which compromised insulin production leads to a vicious cycle of detrimental effects on beta cell survival caused by glucoliptoxicity), when thyroid hormone production is compromised, the hypothyroidism itself does not itself limit compensatory thyroid gland expansion. Normally, the thyroid gland synthesizes thyroid hormone, which is essential for control of metabolism, development, and brain function. A limited number of selectively-expressed thyroid gene products are involved in thyroid hormone production, including thyroglobulin (Tg). The thyroid can devote up to 50% of total protein synthesis to this one protein. Cells such as thyrocytes have a supercharged protein secretion pathway with tonic physiological ER stress. At least 50 Tg mutations are responsible for autosomal recessive congenital hypothyroidism - all of these produce proteins entrapped within the ER. Many Tg mutations are associated with goiter, but for others, compensatory expansion of the thyroid gland is blocked. We hypothesize that for the latter group of Tg mutants, proteotoxic thyroid cell death limits compensatory tissue expansion. In this application, we provide new mechanistic data supporting this hypothesis, highlighting the thyroid gland as the best in vivo system available in which to study ER stress-mediated endocrine cell failure. Quantifying cell death is straightforward in the thyroid system, and importantly, the loss of compensatory tissue expansion can be easily followed in real time, noninvasively, in living animals. Our Specific Aims for the next 5 years are: 1. To define region-dependent effects of the Tg protein on its transport and proteotoxicity; 2. To explore in vivo therapies that facilitate cell survival in the face of ER overload (from misfolded Tg); and 3. To exploit Tgn-/- mice to examine classical ER stress response in thyroid cell death, and to uncover a previously unidentified precursor protein for T4 synthesis.